In my aforementioned U.S. Pat. No. 5,215,296, there is disclosed a positioning apparatus for a fence or guide which supports a workpiece for movement relative to a rotating cutter that projects upwardly through a horizontal work bed. Such apparatus achieved an accuracy of movement on the order of one 1/1000 (0.001) of an inch, but required a plurality of machined metal components together with six rollers secured to the inner walls of a positioning carriage and a pair of nylon rods mounted on a base element for mounting such positioning carriage on the base element for movement parallel to the work bed. A fine pitched, elongated, manually rotatable lead screw was mounted in the carriage to provide micrometer movement of the carriage, but such lead screw was rotatably supported in the carriage only by two small ball bearings respectively engaging the ends of the lead screw and held in position solely by a small spring imparting an axial force to one of the balls. Since the micrometer movement of the positioning carriage results from manually moving a threaded segment into radial engagement with the lead screw, there were ample opportunities for the lead screw to be laterally shifted or elastically bent by such applied force, thus resulting in a slight error in the longitudinal positioning of the carriage.
Additionally, two separate manually operable devices were required to respectively effect engagement of the lead screw by the threaded segment and the clamping of the positioning carriage in the finally selected longitudinal position for guiding a workpiece into a desired engagement with the rotary cutter.
Those skilled in the art of machine cutting of wood workpieces often have to align the center of the workpiece with the cutting axis. For example, in the forming of wooden boxes having interlocked pins and tails at each of the corners it is recognized that symmetrical placement of such pins and tails can only be accomplished by first positioning the workpiece with its center axis accurately aligned with the axis of the rotary cutter. Prior to this invention, such alignment could only be achieved by manual measurements followed by a plurality of trial cuts.